A polymer electrolyte fuel cell in which a hydrogen-containing fuel gas and oxygen-containing oxidizing gas are supplied to an anode and cathode, respectively, and an electromotive force is generated by bipolar electrochemical reaction is generally constituted of sequentially laminating a bipolar plate, a gas diffusion electrode medium, a catalyst layer, an electrolyte membrane, a catalyst layer, a gas diffusion electrode medium and a bipolar plate. The gas diffusion electrode medium is required to have high gas diffusibility to allow a gas supplied from the bipolar plate to be diffused into the catalyst and high drainage property to drain liquid water generated by electrochemical reaction to the bipolar plate as well as high electrical conductivity to extract generated electric current, and electrode base materials composed of carbon fibers and the like are widely used.
However, the following problems are known: (1) when such a polymer electrolyte fuel cell is operated at a relatively low temperature of below 70° C. in a high current density region, as a result of blockage of the electrode base material by liquid water generated in a large amount and shortage in the fuel gas supply, cell performance is impaired (this problem is hereinafter referred to as “flooding”); (2) when such a polymer electrolyte fuel cell is operated at a relatively low temperature of below 70° C. in a high current density region, as a result of blockage of gas flow channels (hereinafter, referred to as “flow channel”) of the bipolar plate by liquid water generated in a large amount and shortage in the fuel gas supply, cell performance is momentarily impaired (this problem is hereinafter referred to as “plugging”); and (3) when such a polymer electrolyte fuel cell is operated at a relatively high temperature of 80° C. or higher, as a result of drying of the electrolyte membrane due to water vapor diffusion and a reduction in the proton conductivity, cell performance is impaired (this problem is hereinafter referred to as “dry-out”). Various efforts have been made to solve these problems of (1) to (3).
JP 2000-123842A proposes a gas diffusion electrode medium in which a microporous region composed of a carbon black and a water-repelling resin is formed on the catalyst layer side of an electrode base material. According to a fuel cell comprising this gas diffusion electrode medium, since the microporous region has a fine pore structure having water repellency, drainage of liquid water of the cathode side is facilitated so that flooding tends to be inhibited. In addition, since generated water is forced back to the electrolyte membrane side (hereinafter, this phenomenon is referred to as “back-diffusion”), the electrolyte membrane is wetted and the problem of dry-out thus tends to be inhibited.
JP 2008-059917A proposes a gas diffusion electrode medium in which a microporous region composed of a scale-like graphite, a carbon black and a water-repelling resin is formed on the catalyst layer side of an electrode base material. According to a fuel cell comprising this gas diffusion electrode medium, since drainage property and water repellency can be adjusted by a scale-like graphite, so that flooding and dry-out tend to be inhibited.
In JP H9-245800A and JP 2008-293937 A, fuel cells comprising a gas diffusion electrode medium in which a microporous region composed of a carbon black and a hydrophobic resin is formed on both sides of an electrode base material are proposed. According to these fuel cells comprising the gas diffusion electrode medium, since the microporous region on the bipolar plate side is smooth and has high water repellency, the flow channel is unlikely to retain liquid water, so that plugging tends to be inhibited. In addition, as a result of facilitation of back-diffusion of a water content to the electrolyte membrane by the microporous region formed on the catalyst layer side and inhibition of water vapor diffusion by the microporous region formed on the bipolar plate side, the electrolyte membrane is wetted and the problem of dry-out tends to be inhibited.
However, in JP 2000-123842A and JP 2008-059917A, there is a problem that flooding and dry-out are still not adequately inhibited and plugging is not improved at all.
Furthermore, in JP H9-245800A and JP 2008-293937 A, there is a problem that prominent flooding occurs because drainage of water from the electrode base material to the bipolar plate is inhibited by the microporous region on the bipolar plate side.
As described above, a variety of technologies have been proposed. However, a gas diffusion electrode medium having excellent anti-flooding and anti-plugging characteristics as well as excellent anti-dry-out characteristic is yet to be discovered.
Therefore, it could be helpful to provide a gas diffusion electrode medium excellent in its anti-flooding, anti-plugging and anti-dry-out characteristics and capable of exerting high cell performance across a wide temperature range from low to high temperatures and has excellent mechanical properties, electrical conductivity and thermal conductivity.